Electroplating cell with hydrodynamics facilitating more uniform deposition on a workpiece with through holes during plating

ABSTRACT

A method and apparatus for establishing more uniform deposition across one or more faces of a workpiece in an electroplating process. The apparatus employs eductors in conjunction with a flow dampener member and other measures to provide a more uniform current distribution and a more uniform metal deposit distribution as reflected in a coefficient of variability that is lower than conventional processes.

CROSS-REFERENCE

This application is a divisional of U.S. application Ser. No. 11/836,903filed Aug. 10, 2007 which is a continuation-in-part of U.S. applicationSer. No. 10/804,841 filed Mar. 19, 2004.

GOVERNMENT RIGHTS

This application was developed under National Science Foundation SmallBusiness Innovative Research Grant No. IIP-0944707.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for controllingthe hydrodynamics in a plating cell to facilitate more uniformdeposition across a workpiece such as a printed circuit board.

BACKGROUND OF THE INVENTION

The continuing miniaturization of electronic devices is driving thedesign of interconnects in the direction of finer pitch surface tracks,smaller diameter through holes and vias, and thicker workpieces toprovide increased circuit densities (Paunovic, M. and M. Schlesinger,2000)¹. This trend has significant implications for the electronicsindustry which must ensure that the metal electrodeposition processmeets the functionality and quality requirements of these advancedworkpiece device designs. These workpieces include printed circuitboards, chip scale packages, wafer level packages, printed wiringboards, high density interconnect printed wiring boards, high densityinterconnect printed circuit boards and the like and these workpiecesoften have at least one through hole extending from a first surface ofthe workpiece to a second surface of the workpiece.

For economies of production, the range of approximate dimensions ofworkpieces is typically 6 inch by 6 inch, 10 inch by 18 inch, 18 inch by24 inch, 2 meters by 2 meters, 5 meters by 5 meters, and 200 millimetersand 300 millimeters in diameter. However, these range of dimensions arenot unique and are not limiting to the need for controlling thehydrodynamics in a plating cell to facilitate uniform deposition acrossa workpiece.

As the hole and via diameter decrease, the workpiece thicknessincreases, and the workpiece dimension increases, the most notablechallenge for the quality of metal electrodeposits is the avoidance ofnon-uniform copper thickness distribution over board surfaces and withinthrough holes, i.e. the challenge of leveling or throwing power, whichcan adversely affect the performance of the finished printed wiringboard interconnect (Paunovic, M. and M. Schlesinger, 1998)², (Ward, M.,D. R. Gabe and J. N. Crosby, 1999a)³.

A number of operating parameters and plating cell attributes influencethe uniformity of copper deposition onto a workpiece. This inventionconcentrates on the influence of electrochemical cell configuration onthe uniformity of copper deposition on the board surface, in particular,the influence of cell configuration on solution hydrodynamics, and theability to generate uniform flow of electrolyte across the surface ofthe board during the plating operation. FIG. 1 shows a plating cell(100) which contains a workpiece (102). Although only one workpiece isshown in this and subsequent drawings, one skilled in the artunderstands that in actual practice a plurality of workpieces may becontained in the plating cell. For ease of description, the termworkpiece is understood to encompass one or more workpieces. Theworkpiece (102) in prior art FIGS. 2-3, 5, and 7-9 is presented as agenerally flat panel having at least one generally flat surface forelectroplating. Arrows (104) indicate the desired uniform flow ofelectrolyte across the entire surface of the workpiece (102).

FIG. 2 shows a conventional workpiece (102—shown in a side-view relativeto its appearance in FIG. 1) plating operation, in which flow ofelectrolyte is achieved by air sparging. Air bubbles (106) are createdin the electrolyte by blowing air through pipes (108) which have holesin them. These pipes are positioned on the bottom of the plating cell(100) beneath the workpiece (102). The number of pipes (108) is notlimited. The movement of air bubbles (106) from the bottom to the top ofthe plating cell (100) creates solution movement, as indicated by thearrows (104). However, air sparging can create problems in the platingoperation:

-   -   the oxygen can oxidize components of the electrolyte,    -   the oxygen can oxidize features and circuit patterns on the        workpiece,    -   air bubbles (106) may become trapped in features in the        workpiece (102), creating areas where copper cannot be        deposited,    -   this method can generate low solution movement rates, which can        result in burning of the workpiece (102) at high current        densities, and    -   as the air bubbles progress towards the top of the cell they        grow in size and can create a non-uniform solution environment        from the bottom of the workpiece to the top.

To avoid the problems associated with air sparging, eductors are beingtested for use in plating cells designed for workpieces. Eductors arenozzles which utilize venturi effects to provide up to five times thesolution flow velocity output of the pump which feeds the eductors.Eductors may be obtained commercially from a number of sources; one sucheductor is marketed under the name Serductor™ (Serductor™ is a trademarkof Serfilco, Northbrook, Ill.)⁴.

One configuration of a prior art plating cell is shown in FIG. 3. Theplating cell (100) contains a workpiece (102) which hangs on a rack(110). Anodes (112) are positioned on either side of the workpiece (102)and hang from rails (114). The workpiece (102) serves as the cathode.Eductors (116) are positioned behind the anodes (112) horizontallyopposite (perpendicular to) the surface of the workpiece (102) (Weber,A., 2003)⁵. Fluid flow is directed (shown by the arrows (104)) from theeductors (116) between the anodes (112) to the surface of the workpiece(102). This type of eductor arrangement leads to impinging fluid flowwhereby the solution flow velocity is directed toward the workpiece.Solution flow velocity is accomplished through the anodes by openings orspaces in the anodes.

However, as shown in FIG. 4, the use of eductors (116) can lead to avariation in solution flow velocity across the workpiece (102) (Chin,D-T. and C-H. Tsang, 1978)⁶, (Hsuch, K-L. and D-T. Chin, 1986a)⁷,(Hsuch, K-L. and D-T. Chin, 1986b)⁸. Fluid flows from the eductor (116)to the impingement point (118) on the surface of the workpiece (102).The fluid flow profile (120) and jet centerline (122) are shown. Theflow from the eductor (116) is directly perpendicular to the surface ofthe workpiece (102). In region I, referred to as the potential coreregion, the flow from the eductor (116) mixes with the surroundingelectrolyte. In region II, referred to as the established flow region,the velocity profile (124) is well established, and the solution flowvelocity decreases as a function of distance from the eductor (116). Inregion III, referred to as the stagnation region, the velocity decreasesto almost zero, and the boundary layer thickness is relativelyindependent of the radial position near the impingement point (118) andcenterline (122). In region IV, referred to as the wall jet region, theradial velocity decreases and the boundary layer thickness increases, asa function of distance radially outward from the impingement point(118). These variations in solution flow velocity, termed the glancingeffect, within regions III and IV contribute to variations in thethickness of copper deposited on the surface of the workpiece (102).

Efforts to improve the uniformity of flow under the impinging eductorflow configuration have included movement of the workpiece (102) whilemaintaining the same distance between the workpiece and the eductor(116). While the workpiece movement has generally been reported as leftand right, the workpiece movement could conceivably be up and down oreven at an angle while maintaining the same distance relative to theeductor. The goal of such movement is to produce a time-averaged uniformboundary layer across the workpiece (102). Such movement, particularlyleft and right movement is termed knife edge agitation by those skilledin the art. However, knife edge agitation still can result innon-uniformity of the deposited copper and adds complexity to platingcell design. Furthermore, incorporation of knife edge movement inexisting workpiece plating lines is difficult and costly.

An alternative prior art configuration shown in FIG. 5 positions theeductors (116) below and off to either side of the workpiece, pointingobliquely at the workpiece surface (102) (Ward, M., D. R. Gabe, and J.N. Crosby, 1998)⁹, (Ward, M., D. R. Gabe, and J. N. Crosby, 1999b)¹⁰.

However, as shown in FIG. 6, the use of angled eductors (116) can leadto a variation in solution flow velocity across the workpiece (102)(Chin, D-T., and M. Agarwal, 1991)¹¹. Fluid flows from the eductor (116)to the impingement point (118) on the surface of the workpiece (102).The fluid flow profile (120) and jet centerline (122) are shown. Theflow from the eductor (116) is at an oblique angle to the surface of theworkpiece (102). In region I, the potential core region, the flow fromthe eductor (116) mixes with the surrounding electrolyte. In region II,the established flow region, the velocity profile (124) is wellestablished, and the solution flow velocity decreases as a function ofdistance from the eductor (116). In region III, the stagnation region,the velocity decreases to almost zero, and the boundary layer thicknessis relatively independent of the radial position near the impingementpoint (118) and centerline (122). In this case, the stagnation point isshifted from the jet centerline. In regions IV and V, the wall jetregions, the velocity decreases and the boundary layer thicknessincreases, as a function of distance radially outward from theimpingement point (118). Furthermore, the solution velocity and boundarylayer thickness in region IV is different from that in region V. Theglancing effect produces variations in solution flow velocity withinregions III, IV, and V, and contributes to variations in the thicknessof copper deposited on the surface of the workpiece (102).

An alternative configuration shown in FIG. 7 positions the eductors(116) below and off to either side of the workpiece, pointing obliquelyacross the workpiece (102) (Weber, A., 2003)⁵. The eductors (116) on oneside of the workpiece (102) are pointed in one direction, and in theopposite direction on the other side (not shown in FIG. 7) of theworkpiece (102). This is intended to create a swirling solution movementaround the workpiece (102). However, the glancing effect described aboveapplies in this case, leading to non-uniform flow of solution across theworkpiece (102).

An alternative configuration shown in FIG. 8 positions the eductors(116) directly below the workpiece (102), pointing directly up so thatsolution moves past the surface of the workpiece (102) (Weber, A.,2003)⁵ (Carano, M., 2003)¹². Again, the glancing effect described aboveapplies in this case, due to mixing of the flow profiles from themultiple eductors (116) positioned below the workpiece (102). Thiscontributes to non-uniform flow of solution across the workpiece (102).

An alternative configuration shown in FIG. 9 positions the eductors(116) directly below and off to either side of the workpiece (102),pointing directly up so that solution moves past the surface of theworkpiece (102) (Carano, M., 2003)¹². The glancing effect describedabove applies in this case, contributing to non-uniform flow of solutionacross the workpiece (102).

Accordingly, a need exists for a method and apparatus which controls thehydrodynamics within a plating cell (100), to facilitate uniformdistribution of metal onto a workpiece (102). This inventionconcentrates on the influence of cell configuration on the uniformity ofdeposition across the surface of the workpiece (102) as reflected in alow coefficient of variability.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for controllingthe hydrodynamics in an electroplating cell (hereinafter called aplating cell), to facilitate a more uniform metal deposit distributionacross the workpiece using an electrochemical plating process, whereinthe metal deposit may be any metal of interest including but not limitedto copper, gold, nickel, tin, lead-tin solder. More uniform depositionis a product of more uniform current distribution which is achieved atleast in part from more uniform solution flow velocity across theworkpiece. Uniform deposition is observed in a coefficient ofvariability (CoV) that is low by industry standards. In accordance withcertain embodiments of the invention CoV less than about 10% and in manycases less than about 7% and in many cases on the order of about 5% orless is achieved.

One embodiment of the present invention more particularly relates tocontrolling the hydrodynamics in a plating cell, to facilitate uniformmetal deposit distribution across a chip scale package using anelectrochemical plating process.

Another embodiment of the present invention relates to controlling thehydrodynamics in a plating cell, to facilitate uniform metal depositdistribution across a wafer level package using an electrochemicalplating process.

Still another embodiment of the present invention relates to controllingthe hydrodynamics in a plating cell, to facilitate uniform metal depositdistribution across a printed wiring board using an electrochemicalplating process.

Another embodiment of the present invention particularly relates tocontrolling the hydrodynamics in a plating cell, to facilitate uniformmetal deposit distribution across a high density interconnect printedwiring board using an electrochemical plating process.

Still another embodiment of the present invention particularly relatesto controlling the hydrodynamics in a plating cell, to facilitateuniform metal deposit distribution across a high density interconnectprinted circuit board using an electrochemical plating process.

Still another embodiment relates to controlling the hydrodynamics in aplating cell to facilitate metal deposition on the walls of a throughhole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the cross-section of a platingcell containing a workpiece, with arrows showing the desired uniformsolution flow velocity across the surface of the workpiece.

FIG. 2 is a schematic illustration of a prior art cell depicting thecross-section of a plating cell containing a workpiece and two anodes,with air sparging.

FIG. 3 is a schematic illustration of prior art depicting thecross-section of a plating cell containing a workpiece and two anodes,with horizontal eductors.

FIG. 4 is a schematic illustration of the velocity profile of the flowfrom an eductor directed onto the surface of a workpiece when theeductor is perpendicular to the workpiece in a prior art cell.

FIG. 5 is a schematic illustration of prior art depicting thecross-section of a plating cell containing a workpiece and two anodeswith angled eductors.

FIG. 6 is a schematic illustration of the velocity profile of the flowfrom an eductor directed onto the surface of a workpiece, when theeductor flow is at an angle which is not 90° with respect to theworkpiece in a prior art cell.

FIG. 7 is a schematic illustration of prior art depicting thecross-section of a plating cell containing a workpiece and two anodes,with angled eductors.

FIG. 8 is a schematic illustration of prior art depicting thecross-section of a plating cell containing a workpiece and two anodes,with vertical eductors directly below the workpiece.

FIG. 9 is a schematic illustration of prior art depicting thecross-section of a plating cell containing a workpiece and two anodes,with vertical eductors below and to either side of the workpiece.

FIG. 10 is a schematic illustration of the cross-section of a platingcell in accordance with one embodiment of the present invention, viewedfrom the top, for controlling hydrodynamic flow within the plating cell,to enhance uniformity of electrochemical deposition of copper onto aworkpiece. This figure shows the flow of electrolyte from the eductorsto the workpiece, and out through a hole in a baffle in the platingcell, to a side chamber, the anodes and anode chambers, the porous fibercloth on the anode chambers, and the workpiece. The non-conductingshielding on the anode chamber is not shown in FIG. 2 so that the porousfiber cloth can be seen in the figure.

FIG. 11 provides another schematic illustration of the plating cellshown in FIG. 10, viewed from a side along the direction 11-11 of FIG.10. Attributes of the plating cell shown in this figure includeeductors, vertical vibration, oscillation perpendicular to the face ofthe panel workpiece, anode to workpiece distance, use of an anodechamber, use of a porous fiber cloth across the front of the anodechamber, anode non-conducting shielding and use of a baffle.

FIG. 12 provides another schematic illustration of the plating cellshown in FIGS. 10 and 11 viewed from a side along the direction 12-12 ofFIG. 10. This figure shows the flow of electrolyte through the eductors,vertically up past the workpiece, and into a side chamber, from where itis pumped back to the eductors. The anodes and anode chambers are notshown in this view.

FIG. 13 is a schematic illustration of the thirty-six thicknessmeasurement points on the copper foil, pulled off the stainless steelpanel used in the plating experiments described in Examples 2 and 3.

FIG. 14 is a set of graphs showing the effects of changing theattributes in the plating cell on the uniformity of metal deposition ona flat stainless steel panel. The smaller the coefficient of variance(CoV), the more uniform the metal deposition

FIG. 15 is a schematic illustration of a plating cell depicting uniformelectrolyte flow across both surfaces of the workpiece.

FIG. 16A is a schematic illustration depicting a plating cell in whichthe electrolyte flows across the first surface of the workpiece at aflow velocity greater than the flow velocity of the electrolyte flowingacross the second surface of the workpiece.

FIG. 16B is a schematic illustration depicting a plating cell in whichthe electrolyte flows across the first surface of the workpiece at aflow velocity less than the flow velocity of the electrolyte flowingacross the second surface of the workpiece.

FIG. 17A is a schematic illustration depicting a plating cell in whichthe electrolyte is injected across the first surface of a workpiecehaving at least one through hole at a flow velocity greater than theflow velocity of the electrolyte injected across the second surface ofthe workpiece. The illustration depicts a workpiece with one throughhole, but there could be a plurality of through holes in the workpiece.Additionally, it is shown that the electrolyte is drawn through thethrough hole from the second surface of the workpiece to the firstsurface by the flow velocity difference.

FIG. 17B is a schematic illustration depicting a plating cell in whichthe electrolyte is injected across the first surface of the workpiece ata flow velocity less than the flow velocity of the electrolyte injectedacross the second surface of the workpiece. Additionally, it is shownthat the electrolyte is drawn through the through hole from the secondsurface of the workpiece to the first surface by the flow velocitydifference.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawing. which form a part hereof, and in which areillustrated specific embodiments in which the invention may bepracticed.

Those skilled in the art will recognize that the invention is notlimited to the specific embodiments illustrated in these drawings. Inthe drawings, the following parts have been identified by the followingnumbers.

100. Plating cell 101. First surface of workpiece 102. Workpiece 103.Second surface of workpiece 104. Arrow indicating electrolyte flow104(a). Arrow indicating high-velocity electrolyte flow 104(b). Arrowindicating low-velocity electrolyte flow 105. Arrow indicating directionof electrolyte flow within a through hole 106. Air bubbles 107. Throughhole 108. Pipe 110. Rack 112. Anode 114. Rail 116. Eductor 118.Impingement point 120. Fluid flow profile 122. Jet centerline 124.Velocity profile 126. Anode chamber 128. Porous fiber cloth 130.Non-conducting shielding 132. Pump 134. Manifold 136. Guide 138. Baffle140. Arrow indicating electrolyte flow 142. Arrow indicating electrolyteflow 144. Hole 146. Baffle 148. Side chamber 150. Outlet hole 152. Arrowindicating vertical vibration 154. Arrow indicating oscillation 156.Copper foil 158. Measuring point

FIGS. 10 to 12 show one embodiment of the present invention in a seriesof cross-sectional views. The following detailed description of thepreferred embodiments refers to these figures. The plating cell (100)was designed with a range of attributes for enhancing the uniformity ofdeposition over the workpiece (102). The attributes are variable highvelocity eductor-induced agitation, lateral oscillation of the workpieceperpendicular its face, use of an anode chamber, variable anode toworkpiece distance, variable frequency vertical vibration of theworkpiece, and non-conducting shielding of the anodes within the anodechamber. In the plating cell (100), the workpiece (102) serves as thecathode for metal deposition.

The plating cell (100), which, in one embodiment, holds 1700 liters ofbath electrolyte is capable of accommodating one rack or workpieceholder (110) which holds one workpiece (102). In this embodiment, theworkpiece is 18 inches by 24 inches high. The size of the plating cell(100), the number of workpieces (102), dimensions of the workpiece (102)and other specific details given here relate to a particular embodimentthat was evaluated experimentally and are not limiting.

Sets of anodes (112) are hung on rails (114, FIGS. 10 and 11) on eachside of the rack (110) and facing the workpiece (102), and may beencased in an anode chamber 126 (FIGS. 10 and 11). These anodes may beplates or, more typically, they are a panel of metallic balls. The anodechamber (126) may have a porous fiber cloth (128) between the anodes(112) and workpiece (102). The cloth may be formed from a polymericmaterial. This cloth (128) spreads the current distribution between theanodes (112) and the workpiece (102) such that the anode chamber (126)acts as a virtual anode. One cloth was obtained from CROSIBLEFILTRATION, located in Moravia, N.Y. 13118. It was specifically a 100%polypropylene filter material. The reported porosity was 2-4 cubic feetper minute. Other filter cloth available has a porosity of 20-30 cubicfeet per minute. A wide variety of filter cloths would be acceptableprovided they have pores small enough that for the given distancebetween the cloth and the workpiece the cloth serves as a virtual anode.

Non-conducting shielding (130) at the top of the anode chamber (126)prevents edge effects from affecting the uniformity of copper depositionon the workpiece (102). The distance from the anode chamber (126) to theworkpiece (102) is adjustable, with a range varying from about 165 to300 mm, and preferably from about 210 mm to 250 mm, and more preferablyfrom about 210 to 220 mm.

In one embodiment two 300 L/min pumps (132) are used to circulateelectrolyte through manifolds (134) on either side of the plating cell(100) and through eductors such as ½ in eductors (116) locatedhorizontally under the anode chambers (126). In FIGS. 10-12, threeeductors (116) are shown on each side of the plating cell (100). In oneembodiment, the eductors are spaced on 6 inch centers, but the number ofeductors (116) and the spacing may change from those cited and is notlimiting. The number and placement of eductors (116) should be chosen soas to facilitate uniform flow of electrolyte across the entire surfaceof the workpiece (102) as described herein.

Electrolyte flowing out of the eductors (116) is directed verticallypast the workpiece (102) by a solution flow velocity dampening member(136), whereby the variations in electrolyte solution are suppressed. Inone embodiment of the invention, the solution flow velocity dampener isa series of shaped guides (136) located below the workpiece (102). Theuse of the shaped guides (136) directs the solution flow parallel thesurface of the workpiece thereby dampening the variations in solutionflow velocity described above in the prior art, reducing the glancingeffect, and resulting in more uniform flow across the surface of theworkpiece (102). The solution flow velocity dampening members that areuseful herein may have a variety of shapes. For example, curved panelsections with various radii of curvature relative to the surface of theworkpiece and flat ramps with various incline angles relative to thesurface of the workpiece. As taught herein, the optimum configurationfor the shaped dampening member is easily determined without undueexperimentation by those of ordinary skill in the art. The radius ofcurvature utilized for one embodiment was 8.25 inches. A useful rangemay be about 6 to 12 inches for a plating cell in which the distancebetween the bottom of the shaped guide and the workpiece isapproximately 10.5 inches.

Baffles (138, FIG. 11) below each anode chamber (126) prevent solutionfrom flowing back to the other side of the anode chamber (126). Thevelocity of the electrolyte flowing past the workpiece (102) can bechanged by 1) changing the pump (132) settings and 2) moving the anodechambers (126) closer to the workpiece (102). The electrolyte flowsvertically up (indicated by arrows 104) past the workpiece (102) andthen across (indicated by arrows 140) the top of the plating cell (100)and out (indicated by arrow 142) through a hole (144) in a baffle (146)in the plating cell (100) to a side chamber (148). Solution is suctionedthrough outlet holes (150) from the side chamber (148) through thepump(s) (132) and back through the manifolds (134) and out through theeductors (116). The side chamber (148) with its enclosed electrolyte andin conjunction with pump(s) (132) and manifold (134) serves as anelectrolyte supply system. In one embodiment, as electrolyte is pumpedthrough the eductors (116), electrolyte in the plating cell (100) ispulled into the eductors (116) in about a 4:1 ratio (4 parts electrolytepulled into the eductors (116) from the plating cell (100) to 1 partelectrolyte pumped through the eductors (116)) to increase the flow ofelectrolyte past the workpiece (102). A filter (not shown) in the sidechamber (148) can be used to maintain cleanliness of the electrolyte.

In some cases, uniformity of metal distribution over the workpiece (102)can be improved by vibration of the workpiece (102). Vibration is in thevertical direction as shown by the double-ended arrow (152) adjacent tothe rack (110) in FIG. 11. Vibration may be particularly important forworkpieces with interconnect features such as fine pitch surface tracks,through holes, vias and the like. Vibration of the workpiece (102) isaccomplished by two horizontally mounted rotary eccentrically weighteddevices powered by variable speed motors (not shown) and mounted to eachend of the load bar (not shown) to which the rack (110) is attached.Those skilled in the art understand that other means for accomplishingvibration include, but are not limited to; pneumatic rotary ball device,pneumatic rotary turbine device, electromagnetic linear motion device,pneumatic sliding piston device, and ultrasonic electromagnetic device.The frequency of vibration available using this configuration typicallyranges from about 0 to 3570 cycles per minute.

Oscillation of the workpiece (102) perpendicular to the anodes (112), asshown by the double-ended arrow (154) above the rack (110) in FIG. 11,or oscillation of the anodes (112) perpendicular to the workpiece (102)or oscillation of both anodes (112) and workpiece (102) with respect toeach other results in the flow of electrolyte through the holes in theworkpiece (102), improving the current distribution and therefore platedmetal distribution on the workpiece (102). Oscillation may beparticularly important for workpieces with interconnect features such asfine pitch surface tracks, through holes, vias and the like. In oneembodiment, oscillation of the workpiece (102) is produced by a positivedrive from a variable speed motor-reducer with crank arm and linkage(not shown). Those skilled in the art understand that other means foraccomplishing oscillation include, but are not limited to reversing rackand pinion device, off axis side crank device, grooved cam traversemechanism device, yoke strap eccentric circular cam mechanism device,reversible worm screw jack device, electromechanical linear drivedevice, and reversible pneumatic or hydraulic cylinder device. Thefrequency of oscillation can shift from about 6 to 63 cycles per minutewith a stroke of about 25 mm, although this range is not limited. Thisis the method of oscillation employed in this plating cell (100),although the invention is not limited to this method. Thus in oneembodiment, the workpiece (102) is moved parallel to and/orperpendicular to the anodes (112).

In accordance with certain embodiments of the invention, uniformity ofmetal distribution over the workpiece (102) can be improved by changingthe distance between the anodes (112) and the workpiece (102). Thedistance from the anode chamber (126) to the workpiece 102 may vary fromabout 165 to 300 mm, preferably from about 210 mm to 250 mm, and morepreferably from about 210 to 220 mm.

Uniformity of metal distribution over the workpiece (102) can also beimproved by placing non-conducting shielding (130) at the top of theanode chamber (126) to reduce edge effects.

Uniformity of metal distribution over the workpiece (102) can be furtherimproved by placing a baffle (138) at the bottom of the anode chamber(126).

The invention is particularly useful in plating circuit boards havingfeatures such as through holes and vias. Because more uniform depositionis available in accordance with the invention, good plating of thefeatures can be achieved independently of the location of the feature onthe workpiece. Thus workpiece having more demanding features to platecan be successfully processed substantially independently of thelocation of the feature on the workpiece. Problems associated withuneven deposition due to uneven boundary layer due to uneven platingsolution flow are minimized and a robust plating technique is provided.

In electroplating methods in which the electrolyte solution is injectedparallel to the surfaces of a workpiece, a equal flow velocity (104) maybe applied across both the first surface (101) and the second surface(103) of the workpiece (102) as shown in FIG. 15. Another embodiment ofthe invention enables plating within through holes is illustrated inFIGS. 16A-B and FIGS. 17A-B. The through hole (107) is representative ofone or more through holes. It is understood in the art that theworkpiece can have a plurality of through holes. In FIG. 16A, the flowvelocity of the electrolyte is adjusted in such a manner that a greaterflow velocity (104 a) is applied across the first surface (101) of theworkpiece than the flow velocity (104 b) applied across the secondsurface (103). FIG. 16B shows that, alternately, the high-velocityelectrolyte flow (104A) can be applied across the second surface (103)and the low-velocity electrolyte flow (104 b) can be applied to thefirst surface (101).

Although not bound by theory, different flow velocities across the firstand second surfaces will generate flow through the through hole asdescribed below. This flow velocity differential will create a highfluid pressure on the surface of the workpiece with the lower flowvelocity (104 b) and will create a low fluid pressure on the firstsurface (101) having the higher flow velocity (104 a). This pressuredifferential will induce flow (105) within the through holes (107),openings that extend throughout the width of the workpiece (FIGS.17A-B). This will cause the plating bath solution from the high-pressureside to flow through the hole and towards the low-pressure side of theworkpiece. This method enables metal deposition within the through holesby providing fresh plating bath and by sweeping away any accumulatedby-products.

A further embodiment of the invention is the use of two or more pumps tomodulate the flow velocity of the electrolyte bath solution. The firstpump can be used to inject electrolyte bath solution into eductors setupto direct the flow via the shaped guides across the first surface of theworkpiece. The second pump can be used to inject bath solution intoeductors set up to direct the flow via the shaped guides across thesecond surface of the workpiece. The first and second pumps can eitherhave controls to regulate their speed (RPM) or have fixed speedcapability. Whether variable or fixed, the pumps should be operated suchthat when utilized in unison they set up a flow velocity differentialacross either side of the workpiece. In addition, a fixed and a variablepumping devices can be used together, once again, provided that theiroperation generates the desired flow velocity differential.

Another embodiment of the invention is the use of valves as a means ofmodulating flow velocity. In this embodiment valves can be inserted inbetween the pump and the eductors. The degree of closure of the valvesaffects the flow velocity of the electrolyte solution prior to enteringthe eductors. Either a single or multiple pumps may be used inconjunction with the valves. Using a single pump, the conduit from thepump can be bifurcated into two separate conduits: a first conduit forinjecting electrolyte bath solution through eductors that direct thesolution via the shaped guides across the first surface of a workpieceand a second conduit for directing the bath solution via the shapedguides across the second surface. Valves can be placed in either one orboth of the conduits. The valves can be adjusted such that the requisiteflow differential is established. Multiple pumps can be utilized in thesame way as described in the previously stated embodiment with theaddition of valves to provide a further means for regulating the flowvelocity.

An even further embodiment of the invention is the use of one or moreflange connections in the conduits. Just as the valves in the aboveembodiment, the flange connections are placed in between the pump andthe eductors. In between the connections a disc with an orifice can beinserted that restricts the diameter of the conduit. Orifice size alsocontributes to the flow velocity across the workpiece. Therefore theflange connectors can be utilized with one or more pumps to regulate theflow velocity across the workpiece. As in the case of the valve whenusing a single pump, the flange connectors can be placed in either thefirst conduit or second conduit or in both. While the valves may beadjustable and therefore their affect on flow velocity adjustable, theflange connections' affect on fluid velocity is limited by the size ofthe orifice selected. Even so, the flange connections can serve aseither the sole means of regulating flow velocity or as an additionalmeans in conjunction with pumps and valves.

The invention will be illustrated by the following examples, which areintended to be illustrative and not limiting.

Example 1 (Comparison)

This example illustrates the use of the plating cell (100) with airsparging to deposit copper onto a workpiece (102), to demonstrate theprior art.

The experiments were conducted in the plating cell (100) shown in FIG.2. An acid copper sulfate electrolyte containing ˜97 g/L CuSO₄, 210-215g/L of concentrated H₂SO₄, ˜63 ppm CF, and 350 ppm polyethylene glycol(PEG) was used as the copper electroplating bath. As known by thoseskilled in the art, the chloride/PEG acts as a suppressor and is notdifficult to control. The plating bath does not containdifficult-to-monitor/control additives such as brighteners and/orlevelers and hence the bath is considered “additive-free.” The platingbath temperature was maintained in the range of 22 to 25° C.

The initial experiments for plating cell (100) characterization wereconducted on a stainless steel panel (450 mm×600 mm), as a workpiece(102). The copper plating process was controlled by DC current at 25A/ft² (provided by a PE86 dual output rectifier) to obtain a copper filmwith a thickness of about 25 micrometers on both surfaces of thestainless steel panel

After each test, copper foils (156) that plated on both sides of thestainless steel panel workpiece (102) were peeled off to analyze thecopper thickness distribution. FIG. 13 illustrates the position of eachmeasuring point (158) on the copper foil (156). There were thirty-sixequi-spaced measuring points on the foil (156) and the edge points were38 mm away from the foil (156) side. The uniformity of copper depositson the steel panel workpiece (102) surface was defined by the ratio ofthe standard deviation to the average copper thickness, expressed on apercent basis ((σ/ā)×100%), that is, the coefficient of variation(CoV)). The quantity σ is the standard deviation based on the measuringpoints; and ā is the mean thickness that is given by: ā=Σh_(i)/n where nis the number of measuring points and h_(i) is the copper thickness ateach measuring point. For these experiments, n=36. The smaller the valueof the CoV, the more uniformly is current distributed over the steelpanel workpiece (102) surface, and the more uniformly is metaldistributed over the steel panel workpiece (102) surface. The value ofCoV for a conventional workpiece with dimensions of about 450 mm×600 mmin the electronics industry is about 10% to 12% although more typicalvalues may be about 15%.

In this example the CoV value determined from analysis of the copperfoil was 13.99%. The thickness of the copper deposit was measured with amicrometer.

Example 2

This example illustrates the use of the plating cell (100) to depositcopper uniformly onto a workpiece (102), to demonstrate the effects ofthe various attributes of the plating cell (100) of the presentinvention, such as flow rate of electrolyte through the eductors (116),anodes (112) to workpiece (102) distance, oscillation (154) of theworkpiece (102), and vibration (152) of the workpiece (102).

The experiments were conducted in the plating cell (100) shown in FIGS.10 to 12. An acid copper sulfate electrolyte containing ˜97 g/L ofCuSO₄, 210-215 g/L of concentrated H₂SO₄, ˜63 ppm Cl⁻, and 350 ppmpolyethylene glycol (PEG) was used as the copper electroplating bath forall experiments. The chloride/PEG is termed a suppressor and is notdifficult to control. The plating bath does not containdifficult-to-monitor/control additives such as brighteners and/orlevelers and hence we consider the bath as “additive-free.” The platingbath temperature was maintained in the range of 22 to 25° C.

The initial experiments for plating cell (100) characterization wereconducted on a stainless steel panel (450 mm×600 mm), as a workpiece(102). The cell operating parameters, which were eductor (116) flow rate(low flow designates flow with a pump setting about one-half the maximum(high) flow), oscillation (154) frequency, vibration (152) frequency,and anode (112) to steel panel workpiece (102) distance, were selectedas factors to evaluate the effect of plating cell (100) configuration onthe current distribution over the panel workpiece (102) surface. Thecopper plating process was controlled by DC current at 25 A/ft²(provided by a PE86 dual output rectifier) to obtain a copper film witha thickness of about 25 micrometers. In all experiments, the anodechamber (126) was used, as was the porous fiber cloth (128), and 152 mmof anode non-conducting shielding (130).

After each test, copper foils (156) that plated on both sides of thestainless steel panel workpiece (102) were peeled off to analyze thecopper thickness distribution. FIG. 13 illustrates the position of eachmeasuring point (158) on the copper foil (156). There were thirty-sixequi-spaced measuring points on the foil (156) and the edge points were38 mm away from the foil (156) side. The uniformity of copper depositson the steel panel workpiece (102) surface was defined as described inExample 1 above, with n=36 in this example also. The desired percentagevalue of CoV for the cell conditions in the electronics industry andmore particularly printed circuit board industry for panels ofapproximately this size is less than 10%.

The experimental matrix, designed using a full factorial method, islisted in Table 1. MINITAB software was used to design the factorialmethod, although other methods could be used. The target performancecriterion for the initial cell experimental study was to plateapproximately 25 micrometers of copper over the steel panel workpiece(102) surface and evaluate the uniformity of copper thicknessdistribution. As shown in Table 1, a CoV of less than 10% was achievedunder the plating cell operating conditions of Test 5 to Test 12, andthe lowest CoV value was achieved in Test 5.

TABLE 1 Factorial Matrix and CoV Results for Example 2. Test OscillationVibration Distance* CoV No. Flow (cycles/min) (cycles/min) (mm) (%) 1High 26 0 290 12.11 2 High 12 0 290 12.55 3 High 12 1400 290 12.12 4High 26 1400 290 12.35 5 High 26 1400 213 7.72 6 High 26 0 213 8.57 7High 12 1400 213 9.54 8 High 12 0 213 9.19 9 Low 12 0 213 9.76 10 Low 121400 213 8.31 11 Low 26 1400 213 9.01 12 Low 26 0 213 9.54 13 Low 26 0290 11.42 14 Low 26 1400 290 16.12 15 Low 12 1400 290 11.55 16 Low 12 0290 11.13 *Distance refers to the anode-to-workpiece distance.

FIG. 14 shows a graph of the data from the factorial matrix. The graphplots the CoV versus the changes in each of the operating parameters andshows which operating parameter has the strongest influence on theuniformity of copper thickness across the surface of the stainless steelpanel workpiece (102). FIG. 14 shows that the distance between theanodes (112), which controls the anode (112) to steel panel workpiece(102) distance, has the strongest influence on the uniformity of copperdistribution over the steel panel workpiece (102), compared to the otherparameters. However, one skilled in the art would recognize thatoscillation and vibration may be important when the workpieceincorporates interconnects with fine pitch lines, through holes, viasand the like. These data would also indicate that even closer anode(112) to steel panel workpiece (102) spacing may offer furtherimprovements in copper uniformity.

These observations are confirmed by the data in Table 1 which show thata more uniform copper thickness distribution (low CoV) can be obtainedby using a closer distance between the anode chamber (126) and thestainless steel panel workpiece (102). The Test 5 plating cellconfiguration gave the most uniform copper thickness distribution overthe steel panel workpiece (102) surface, with the closest anode (112) tosteel panel workpiece (102) distance, at a high flow rate, highoscillation frequency and high vibration frequency.

Based on the test results shown in FIG. 14, the effect of oscillation(154) and vibration (152) are unclear, although they suggest that highervibration (152) and oscillation (154) frequencies will improve theuniformity of metal on the steel panel workpiece (102). The effects ofoscillation (154) and vibration (152) might be seen more clearly on apatterned workpiece which has interconnect features such as fine pitchlines, through holes, and vias and the like.

Example 3

This example illustrates the use of the plating cell (100) to depositcopper uniformly onto a workpiece (102), to demonstrate further effectsof the various attributes of the plating cell (100), such as flow rateof electrolyte through the eductors (116), anodes (112) to workpiece(102) distance, use of an anode chamber (126), use of a porous fibercloth (128), use of additional non-conducting shielding (130), and useof a baffle (138) under the anode chamber, on the current distributionover the panel workpiece (102) surface.

The experiments were conducted in the plating cell (100) shown in FIGS.10 to 12. An acid copper sulfate electrolyte containing ˜97 g/L ofCuSO₄, 210-215 g/L of concentrated H₂SO₄, ˜63 ppm Cl⁻, and 350 ppmpolyethylene glycol (PEG) was used as the copper electroplating bath forall experiments. The chloride/PEG acts as a suppressor and is notdifficult to control. The plating bath does not containdifficult-to-monitor/control additives such as brighteners and/orlevelers and hence we consider the bath as “additive-free.” The platingbath temperature was maintained in the range of 22 to 25° C.

The initial experiments for cell characterization were conducted on astainless steel panel (450 mm×600 mm), as a workpiece (102). The copperplating process was controlled by DC current at 25 A/ft² (provided by aPE86 dual output rectifier) to obtain a copper film with a thickness ofabout 25 micrometers.

After each test, copper foils (156) that plated on both sides of thestainless steel panel workpiece (102) were peeled off to analyze thecopper thickness distribution. FIG. 13 illustrates the position of eachmeasuring point (158) on the copper foil (156). There were thirty-sixequi-spaced measuring points on the foil (156) and the edge points were38 mm away from the foil (156) side. The uniformity of copper depositson the steel panel workpiece (102) surface was defined as described inExample 1 above, with n=36 in this example also. The desired percentagevalue of CoV for the cell conditions in the electronics and moreparticularly printed circuit board industry is less than 10%.

The experimental matrix and results are listed in Table 2. The targetperformance criterion for the experimental study was to plateapproximately 25 micrometers of copper over the steel panel workpiece(102) surface and evaluate the uniformity of copper thicknessdistribution.

TABLE 2 Experimental Matrix and CoV Results for Example 3. 5C Same asTest 5 but with 203 mm distance between 9.47 anode and panel 5D Same asTest 5 but with 191 mm additional shielding 5.24 on top of anode chamber5E Same as Test 5 but no anode chamber in cell 14.81 SF Same as Test 5with anode chamber without fiber cloth 11.61 5CG Test 5C conditions,adding baffle under the bottom 8.31 of each anode chamber 5DH Test SDconditions, adding baffle under the bottom 5.18 of each anode chamber5DI Test 5D conditions with fiber cloth (did not dummy 5.45 the bath)5DJ Test 5D conditions with fiber cloth 5.39 11 Low flow, 26 cycles/minoscillation, 1400 cycles/min 9.01 vibration, 213 mm distance betweenanode center and panel, anode chamber with fiber cloth, 152 mm shieldingon top of anode chamber. 11C Same as Test 11 but low flow and 203 mmdistance 10.06 between anode center and panel 11E Same as Test 1.1 butlow flow, no anode chamber in cell 14.11

Table 2 shows the effect of each plating cell attribute. Comparing Test5 with 5C and Test 11 with 11C shows that decreasing the distancebetween the anode (112) and panel workpiece (102) from 213 to 203 mmdecreased the uniformity (increased the CoV) of metal deposition acrossthe steel panel workpiece (102). Comparing Test 5 with 5D shows thatincreasing the non-conducting shielding (130) at the top of the anodechamber (126) from 152 to 191 mm improved the uniformity (decreased theCoV) of metal deposition across the steel panel workpiece (102).Comparing Test 5 with 5E and Test 11 with 11E shows that removing theanode chambers (126) from the cell decreased the uniformity (increasedthe CoV) of metal deposition across the steel panel workpiece (102).Comparing Test 5 with 5F shows that removing the porous fiber cloth(128) from the anode chamber (126) decreased the uniformity (increasedthe CoV) of metal deposition across the steel panel workpiece (102).Comparing Test 5C with 5CG and Test 5D with 5DH shows that adding abaffle (138) under the bottom of each anode chamber (126) improved theuniformity (decreased the CoV) of metal deposition across the steelpanel workpiece (102). Comparing Test 5D with SDI and 5DJ shows thatchanging the porous fiber cloth (128) to that of a differentmanufacturer decreased the uniformity (increased the CoV) of metaldeposition across the steel panel workpiece (102). In summary, the bestresult was achieved in Test 5DH, which ran at high flow, 26 cycles/minoscillation, 1400 cycles/min vibration, 213 mm distance between anode(112) and steel panel workpiece (102), used an anode chamber (126) witha porous fiber cloth (128), had 191 mm non-conducting shielding (130) ontop of the anode chamber (126), and had a baffle (138) attached belowboth anode chambers (126).

The invention having now been fully described, it should be understoodthat it might be embodied in other specific forms or variations withoutdeparting from its spirit or essential characteristics. Accordingly, theembodiments described above are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are intended to be embraced therein.

REFERENCES

-   1 M. Paunovic and M. Schlesinger (2000), Modern Electroplating,    Wiley Inc. NY.-   2 M. Paunovic and M. Schlesinger (1998), Fundamentals of    Electrochemical Deposition, Wiley Inc. NY.-   3 Ward, M., D. R. Gabe and J. N. Crosby (1999a), Proc. European PCB    Convention, Munich, Germany, November.-   4 Serductor™ is a trademark of Serfilco, Northbrook, Ill.-   5 Weber, A. (2003), The Importance of Plating Cell Design and    Hydrodynamics for Repeatable Product Quality in Latest Generation    Vertical Platers for the Galvanic Industry, IPC Printed Circuits    Expo 2003, Long Beach, Calif.-   6 Chin, D-T. and Tsang, C-H. (1978), Mass Transfer to an Impinging    Jet Electrode, J. Electrochem. Soc., 125, 9, pp 1461-1470.-   7 Hsuch, K-L. and D-T. Chin (1986a), Mass Transfer to a Cylindrical    Surface from an Unsubmerged Impinging Jet, J. Electrochem. Soc.,    133, 1, pp 75-81.-   8 Hsuch, K-L. and D-T. Chin (1986b), Mass Transfer of a Submerged    Impinging Jet on a Cylindrical Surface, J. Electrochem. Soc., 133,    9, pp 1845-1850.-   9 Ward, M., D. R. Gabe, and J. N. Crosby (1998), Novel Agitation for    PCB Production: Use of Eductor Technology, Trans IMF, 76, 4, pp    121-126.-   10 Ward, M., D. R. Gabe, and J. N. Crosby (1999b), Exploitation of    Eductor Agitation in Copper Electroplating, Proc. SURFIN/99, June    21-24, Cincinnati, Ohio.-   11 Chin, D-T. and M. Agarwal (1991), Mass Transfer from an Oblique    Impinging Slot Jet, J. Electrochem. Soc., 138, 9, pp 2643-2650.-   12 Carano, M. (2003), Hole Preparation & Metallization of High    Aspect Ratio, High Reliability Back Panels, Part-2, Circuitree,    February, pp 10-22.

1. A plating cell comprising: a holder for a workpiece having first andsecond major surfaces, the workpiece holder defining a plane, an anodeon each side of the plane defined by the workpiece holder, each anodebeing housed in an anode chamber of a size greater than the workpieceholder, a solution flow dampening member on each side of the planedefined by the workpiece holder, the flow dampening members beingcoextensive with the anode chamber on the same side of the plane, aplurality of eductors on each side of the plane for directing anelectrolyte solution onto the flow dampening member on the same side ofthe plane, and a first pump connected to a conduit to supply electrolytesolution to the plurality of eductors on the first side of the plane anda second pump connected to a conduit to supply electrolyte solution tothe plurality of eductors on the second side of the plane to control theflow velocity of the solution over the major surfaces of the workpiece,and a controller for regulating the first pump, second pump or bothpumps wherein the eductors, the dampening members and the workpieceholder are arranged such that electrolyte solution ejected from theeductors is directed by the dampening members to flow across theworkpiece to produce a solution flow that is uniform and parallel toboth the first and second major surfaces of the workpiece; and thecontroller is configured to operate the pumps such that the solutionflow velocity over one of the first and second major surface of theworkpiece is greater than the solution flow velocity over the othermajor surface of the workpiece.
 2. The plating cell of claim 1, furthercomprising a vibrator for the workpiece.
 3. The plating cell of claim 1,further comprising an oscillator for the workpiece.
 4. The plating cellof claim 1, wherein the distance between at least one of the anodes andthe workpiece holder is about 165 to about 300 mm.
 5. The plating cellof claim 1 wherein at least one of the flow dampening members is amember having a curved surface.
 6. The plating cell of claim 1, whereinat least one of the anode chambers contains a porous cloth.
 7. Theplating cell of claim 6, wherein the porous cloth is interposed betweenthe anode and the workpiece holder such that the ionic current flowbetween the anode and the workpiece passes through the porous cloth. 8.The plating cell of claim 7 wherein the plating cell further comprisesat least one valve inserted in the conduit between at least one of saidpumps and the plurality of eductors to which the pump is connected.
 9. Aplating cell comprising: a holder for a workpiece having first andsecond major surfaces, the workpiece holder defining a plane, an anodeon each side of the plane defined by the workpiece holder, each anodebeing housed in an anode chamber of a size greater than the workpieceholder, a solution flow dampening member on each side of the planedefined by the workpiece holder, the flow dampening members beingcoextensive with the anode chamber on the same side of the plane, aplurality of eductors on each side of the plane for directing anelectrolyte solution onto the flow dampening member on the same side ofthe plane, a pump connected to supply electrolyte solution to theplurality of eductors on the first side and the second side of theplane, and a first valve or a first flange inserted in a conduit betweenthe pump and the plurality eductors on the first side of the plane orinserted in a conduit between the pump and the plurality of eductors onthe second side of the plane to control the flow velocity of thesolution over the major surfaces of the workpiece, wherein the eductors,the dampening members and the workpiece holder are arranged such thatelectrolyte solution ejected from the eductors is directed by thedampening members to flow across workpiece to produce a solution flowthat is uniform and parallel to both the first and second major surfacesof the workpiece; and the valve or the flange is configured such thatthe solution flow velocity over one of the first and second majorsurface of the workpiece is greater than the solution flow velocity overthe other major surface of the workpiece.
 10. The plating cell of claim9, further comprising a vibrator for the workpiece.
 11. The plating cellof claim 9, further comprising an oscillator for the workpiece.
 12. Theplating cell of claim 9, wherein the distance between at least one ofthe anodes and the workpiece holder is about 165 to about 300 mm. 13.The plating cell of claim 9 wherein at least one of the flow dampeningmembers is a member having a curved surface.
 14. The plating cell ofclaim 9, wherein at least one of the anode chambers contains a porouscloth.
 15. The plating cell of claim 14, wherein the porous cloth isinterposed between the anode and the workpiece holder such that theionic current flow between the anode and the workpiece passes throughthe porous cloth.
 16. The plating cell of claim 15 wherein the platingcell further comprises a second valve or a second flange insertedbetween the pump and the plurality of eductors.